1. Technical Field
The present invention relates to a mark detection method using a printing apparatus and a printing apparatus employing the mark detection method.
2. Related Art
Some image forming apparatuses (printing apparatuses) known in the art feed a paper sheet (web) disposed in a roll shape to a printing section and form (record) images on this paper sheet. Such printing apparatuses use positioning marks (eye marks) formed on a paper sheet to adjust locations at which images are to be printed (e.g., JP-A-2010-228168).
In an exemplary printing apparatus as described in JP-A-2010-228168, when a web is displaced in its width direction during the transport, the detecting location of a mark sensor follows this web, thus preventing the mark sensor from failing to detect eye marks on the web. More specifically, the printing apparatus locates the centers of an eye mark and a blank area formed in the eye mark in a transport direction of the web on the basis of a detection signal from the mark sensor. Then, the printing apparatus calculates the difference between both center locations in a width direction of the web, determining the shift amount of the eye mark in the width direction. The printing apparatus moves the detecting location of the mark sensor in the width direction by the determined shift amount. In this way, the mark sensor successfully follows the shifted eye marks.
A typical size of the eye marks is as small as 5 mm per side. Therefore, the mark sensor needs to have a high-precision detection capability in order to detect variation in the center locations of the blank areas in the eye marks. Disadvantageously, using a high-precision detection sensor may lead to an increase in the cost of the printing apparatus.
Tiny foreign objects may adhere to the blank areas in some eye marks and affect the calculation of shift amounts of these eye marks. This may make it difficult for the mark sensor to follow the eye marks. Furthermore, the mark sensor has a detection spot that is smaller than each eye mark. If a web is greatly displaced in its width direction, for example at the start of the transport, the mark sensor may fail to detect eye marks. In short, the printing apparatus has trouble detecting the eye marks stably due to adhesion of foreign objects to the eye marks, the displacement of the web at the start of the transport, and the like.